In order to understand the atomic geometry of 16 x 2 superstructure reconstructed on clean Ge(1 1 0) surfaces, a 16 x 2 structural model was proposed, based on scanning tunneling microscopic (STM) observations, and relaxed by using an ab initio structure optimization program. STM images calculated from the relaxed model could well explain features of experimental STM images, which indicates that five-membered clusters of Ge adatoms are entities of pentagons observed in the STM images. (C) 2003 Elsevier B.V. All rights reserved.